The Power of Plain English
Here's some advice from a speech on writing good English by William Zinssler of the Columbia School of Journalism. It was meant for foreign students but it still applies just as much to native speakers. A lot of it involves unlearning what you learned in school.

Repeat after me:
Short is better than long.
Simple is good. (Louder)
Long Latin nouns are the enemy.
Anglo-Saxon active verbs are your best friend.
One thought per sentence.